The validity
of measurements made by industrial sensorsand instruments
is ensured by regular calibration, providing traceability through
to a National Standard. In other words, a field sensor must be
calibrated against a reference system that itself has an un-broken
chain of calibration through to some primary system, such as
operated by National Standards laboratories like NPL in the UK,
NIST in the USA and PTB in Germany.

Alternatively,
large independent companies may operate their own primary standards
in certain measurement fields such as electrical parameters,
pressure etc.

Therefore there
is a need for high quality, reliable and repeatable calibration
reference equipment to effect the link between an industrial
sensor and the primary standard. In the field of humidity calibration,
Michell Instruments has developed a variety of calibration systems
that are suitable for use as a stand-alone facility to provide
a high performance level and act as a transfer-standard between
a primary humidity standard and the field sensor, or some other
reference hygrometer.

Michell has developed
an expertise in providing such humidity calibration equipment
to cover the full humidity range, from trace moisture measurement
at ppm levels through to relative humidity measurement at elevated
temperatures. Each system is built to customers' specific requirements.
Normally a calibration system will comprise an integrated compressor/dryer
to produce a zero gas ( no moisture) feed into a humidity generator
and sensor housing that are designed for the user's specific
needs. The final element of calibration system is always a precision
cooled mirror reference hygrometer that provides direct traceability
to a primary humidity standard.

A good example
is at Justervesenet, the Norwegian standards institute in Oslo.
In the absence of a primary standard within Norway, this institute
decided to use two humidity calibration systems supplied by Michell
Instruments, one covering the range -90 to +20 oC dew point and
the other for the high temperature/high humidity range up to
+80 oC dew point. The S4000 reference hygrometers used in each
system are directly calibrated at NPL.

These systems
provide Justervesenet the capability to calibrate other reference
hygrometers or field humidity sensors with direct traceability
to a primary standard, giving the end user added confidence in
humidity measurement.
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